The invention relates to a method and apparatus for producing a corona discharge, and more particularly to a method and apparatus for producing a uniform corona along at least one stationary elongate corona electrode.
It is well known that in electrostatic printing equipment a corona generating device including a corona discharge electrode is employed to place positive or negative charges onto a photoconductive member or surface.
The generated corona may be either positive or negative in order to establish a corresponding charge, depending upon the nature of the photoconductive surface employed. When a positive corona is generated from a metallic filament electrode, the resultant charge applied to the photoconductive surface is generally relatively uniform due to the intrinsic uniformity of the nature of a positive corona electrode emission. Many of the currently available electrophotographic devices require a negative corona. When a negative corona is generated from a metallic filament electrode, the photoconductive surface obtains a charge which varies in density from point to point due to the nature of the nonuniform negative corona electrode emission. It is believed that this nonuniformity in charge detrimentally manifests in the developed image since areas containing a higher charge will attract more electrostatic developer material thereto, thereby creating a streaked, nonuniform image appearance.
A number of devices have been developed in order to produce a more uniform negative corona. One such device described by Jarvis, et al., U.S. Pat. No. 3,233,156 includes a rotating corona producing electrode in the form of a helix having a high negative D.C. voltage applied thereto. Rotating the helix has the effect of continuously changing the corona generation sites. This device is considerably limited in its ability to charge a photoconductive surface since the corona generation sites are only those points of the rotating helix immediately adjacent the photoconductive surface. The present invention provides an elongate corona electrode that is active along its entire length thus having significantly improved charging ability over the Jarvis device.
Kuehnle U.S. Pat. No. 3,958,162 describes an improved negative corona producing device which includes a number of elongate corona electrodes, which are positioned parallel to and a predetermined distances from a central axis, and rotated about the central axis while simultaneously applying a corona voltage to each electrode. This device interchanges one corona generating electrode for another corona generating electrode in rapid sequence by the rotating action. The effect of the nonuniformities in each elongate corona electrode are substantially offset, and thereby averaged out, by the rotating action that interchanges the several effective corona generating electrodes, however the pulse like interruption of current flow between the photoconductive surface and each corona generating electrode produced thereby limits the average charging ability of the device.
The present invention provides a method and apparatus for uniformly charging a photoconductive surface. The apparatus includes at least one stationary elongate corona electrode and a moving ground electrode, thereby producing a continuous and substantially uniform corona discharge along the entire length of the stationary elongate corona electrode.